That's the implication of a new survey conducted recently in Australia. According to survey respondents, the "happily ever after" endings so common in the genre have created expectations that real relationships fail to meet - such as the belief that one can or should always know what one's partner is thinking, or that they should regularly receive flowers "just because".
Though it's not stated outright, I strongly suspect that most of the respondents to this survey were female, and the survey's findings square with the theory that the typical Hollywood romcom is essentially emotional pornography for women. Just as actual pornography supposedly leads men to nurture unrealistic expectations about their partners and their sex lives, romantic comedies, with their idealized, Hallmark card vision of romantic relationships, so the argument goes, do so for women. I'm not convinced that this is true - I'm skeptical of such sweeping claims, particularly when they're based on survey results that don't appear to have been gathered under particularly scientific conditions -but the thesis is thought-provoking.
So no, crappy middling romantic comedies are not a good basis on which to found one's expectations of love and romance. But it seems to me that most people I know - mature adults, at least - have figured that out, and have (if through painful experience) learned that real relationships sometimes require work, sacrifice, and a very unsexy attention to practical details as well as spontaneity and passion. The best romantic comedies find a way to acknowledge this truth, even if they also try to sell us a romantic fantasy. Lesser ones don't, but that is one of the major reasons they're considered lesser. Are there really people out there who take their cues from Maid In Manhattan or The Ugly Truth? If so, ugh. Romance is in more trouble than we could have guessed.